System for controlling the discharge of drilling fluid

ABSTRACT

A drilling system is provided for drilling a borehole into an earth formation, the drilling system comprising pump means for pumping drilling fluid into the borehole and discharge means for discharging drilling fluid from the borehole. The discharge means comprises at least one pressure chamber for temporarily accommodating drilling fluid being discharged from the borehole, and control means for controlling the fluid inflow into each pressure chamber.

[0001] The present invention relates to a drilling system and a methodfor drilling a borehole into an earth formation, the drilling systemcomprising pump means for pumping drilling fluid into the borehole anddischarge means for discharging drilling fluid from the borehole.

[0002] The drilling system may furthermore comprise a drill stringextending into the borehole whereby an annular space is formed betweenthe drill string and the borehole wall, the annular space containing abody of drilling fluid. The drill string generally has a longitudinalpassage for pumping drilling fluid into the annular space through aopening near the lower end of the drill string. The drilling fluid canbe discharged from the borehole through a discharge conduit connectedwith the borehole near the upper end of said annular space. The flow ofdrilling fluid through said annular space can be controlled by saiddischarge means, for example by a controllable resistance in saiddischarge conduit.

[0003] Therefore the discharge conduit can be provided with a chokevalve providing a controllable throttle opening. However, because ofrock debris and contaminated mud in the drilling fluid a throttleopening in the discharge conduit shall be worn out soon.

[0004] WO-A-0079092 discloses such drilling system, whereby thedischarge means control the discharge of drilling fluid,and therewiththe flow of drilling fluid through the annular space. Therefore thedischarge conduit is provided with a controllable outlet valve. As analternative WO-A-0079092 describes an injection pump arranged to pumpinjection fluid via an injection nozzle into the discharge conduit in adirection opposite to the direction of flow of drilling fluid throughthe discharge conduit. By controlling the injection fluid, theresistance in the discharge conduit can be controlled.

[0005] It is an object of the invention to provide for an improvedsystem and method for controlling the discharge of drilling fluid from aborehole.

[0006] In accordance with the invention there is provided a drillingsystem for drilling a borehole into an earth formation, the drillingsystem comprising pump means for pumping drilling fluid into theborehole and discharge means for discharging drilling fluid from theborehole, wherein the discharge means comprises at least one pressurechamber for temporarily accommodating drilling fluid being dischargedfrom the borehole, and control means for controlling the fluid inflowinto each pressure chamber.

[0007] Thereby it is achieved that the drilling fluid from the boreholeis transported to the pressure chamber and the inflow of it into thepressure chamber can be controlled without a restriction through whichthe drilling fluid has to flow.

[0008] Preferably said control means is arranged to control the fluidpressure in the pressure chamber.

[0009] The inflow of drilling fluid into the pressure chamber can becontrolled by controlling the outflow of gas or liquid which is expelledfrom the pressure chamber by the inflow of drilling fluid. Such gas orliquid, hereinafter referred to as expel fluid, can be led through acontrollable throttle opening, thereby controlling the inflow ofdrilling fluid. In fact thereby the said control means control fluidpressure above the drilling fluid in the pressure chamber.

[0010] In fact the discharge of drilling fluid from the borehole iscontrolled by throttling the expel fluid in stead of throttling thedrilling fluid. And because the expel fluid does not contain rock debrisor contaminated mud, there is no wear problem in the choke valvethrottling the fluid.

[0011] Preferably the pressure chamber is provided with two compartmentsseparated by a flexible membrane, whereby one of the compartments is tobe filled with drilling fluid and the other compartment contains anexpel fluid, whereby said control means control the outflow of saidexpel fluid from the pressure chamber.

[0012] In a preferred embodiment two or more pressure chambers beingalternately filled with drilling fluid from the borehole, whereby saidcontrol means control the inflow of drilling fluid in each of thepressure chambers. By making use of more then one pressure chamber, thedrilling fluid can be removed from a pressure while the drilling fluidfrom the borehole can be led to another pressure chamber.

[0013] Preferably two pressure chambers are interconnected by an expelfluid conduit for transporting an expel fluid between the two pressurechambers, whereby said control means comprise a control valve in saidexpel fluid conduit.

[0014] In a preferred embodiment the discharge means comprise twopressure chambers, each provided with a membrane to form a drillingfluid compartment and an expel fluid compartment, both having a variablecontent, the expel fluid compartments being interconnected by an expelfluid conduit provided with a control valve for controlling flow throughsaid expel fluid conduit, the system furthermore being provided withinlet valve means to direct the drilling fluid to be dischargedalternately to one of said drilling fluid compartments and with outletvalve means to remove drilling fluid from the other drilling fluidcompartment.

[0015] The invention furthermore relate to a method for drilling aborehole into an earth formation, whereby drilling fluid is pumped intothe borehole and whereby drilling fluid is discharged from the boreholeand transported to a pressure chamber, whereby the inflow of drillingfluid into the pressure chamber is controlled.

[0016] The invention will now be described in more detail and by way ofexample with reference to the accompanying drawing in which:

[0017]FIG. 1 schematically shows an embodiment of a drilling system; and

[0018]FIGS. 2 and 3 schematically show the drilling fluid dischargemeans.

[0019] In the figures like reference numerals relate to like components.

[0020] In FIG. 1 is shown a drill string 1 extending into a borehole 3formed in an earth formation 5 and provided with a drill bit 7 and abottom hole assembly (BHA, not shown). The drill string 1 is made up ofa plurality of drill string joints, whereby each pair of adjacent jointsis interconnected by a releasable connector. For the purpose of clarityonly one of the uppermost connectors 9 a, 9 b, which connects theuppermost joint to the remainder of the drill string 1, is shown (indisconnected mode). In the description hereinafter, the upper drillstring joint is referred to as the upper drill string section 10 and theremainder of the drill string 1 is referred to as the lower drill stringsection 12. The lower drill section 12 is supported at rig floor 14 of adrilling rig (not shown) by power slips 16. The upper drill stringsection 10 is supported by a top drive 18 which is capable of supportingthe entire drill string 1 and which is provided with a drive system (notshown) for rotating the drill string 1 during drilling. A primary pump19 is in fluid communication with the upper drill string section to pumpdrilling fluid through the drill string 1 when the connector 9 a, 9 b isin connected mode.

[0021] A fluid chamber 20 is supported by a support column 22 providedat rig floor 14 in a manner allowing the fluid chamber 20 to move up ordown along the column 22, and means (not shown) are provided to controlsuch movement. The upper drill string section 10 extends into the fluidchamber 20 through an upper opening 24 of the fluid chamber 20 so thatthe open lower end of the upper drill string section 10 is located in anupper portion 25 of the chamber 20. The lower drill string section 12extends into the fluid chamber 20 through a lower opening 26 of thefluid chamber 20 so that the open upper end of the lower drill stringsection 12 is located in a lower portion 27 of the chamber 20. Bothupper opening 24 and de lower opening 26 are of a sufficiently largediameter to allow passage of the drill string connectors (whichgenerally are of slightly larger diameter than the drill stringsections) therethrough. Furthermore, the upper and lower openings 24, 26are provided with seals 29 a, 29 b which are controllable so as to bemoved radially inward and thereby to seal against the respective upperand lower drill string sections 10,12. The lower portion 27 of chamber20 is provided with a fluid inlet 28 in fluid communication with asecondary pump 30 to pump drilling fluid through the lower drill stringsection 12 when the connector 9 a, 9 b is in disconnected mode.

[0022] The upper portion 25 and the lower portion 27 of the fluidchamber 20 are selectively sealed from each other by a partitioningmeans in the form of a valve 32. A control device (not shown) isprovided to open or close the valve 32, whereby in its open position thevalve 32 allows passage of drill string 1 through the valve 32.Furthermore, in the open position of the valve 32, the upper portion 25and the lower portion 27 of the fluid chamber 20 are in fluidcommunication with each other. A pair of power tongues 34,36 connectingand disconnecting the connector 9 a,9 b is attached to the fluid chamber20 at the lower side thereof.

[0023] An annular space 38 is defined between the lower drill stringsection 12 on one hand and the borehole wall and a wellbore casing 42 onthe other hand, which annular space is filled with a body of drillingfluid 40. The annular space 38 is at its upper end sealed by a rotatingblowout preventor (BOP) 46 which allows rotation and vertical movementof the drill string 1. A drilling fluid discharge conduit 48 is providedat the upper end of the annular space 38, which discharge conduit 48debouches into a drilling fluid reservoir (not shown) via dischargemeans 50, which discharge means shall be elucidated hereinafterreferring to FIGS. 2 and 3. A tertiary pump 52 is arranged in parallelwith the discharge means 50, which pump 52 is in fluid communicationwith the discharge conduit 48 at a branch connection 54 located betweenthe discharge means 50 and the rotating BOP 46. The pump 52 is operableso as to pump drilling fluid from a drilling fluid reservoir (not shown)into the annular space 38. The lower part of the drill string 1 isprovided with means for controlling the flow of drilling fluid from thebody of drilling fluid 40 into the drill string 1 in the form of anon-return valve (not shown) which prevents such return flow.

[0024] During normal operation the drill string 1 is rotated by the topdrive 18 to further drill the borehole 3 whereby the connector 9 a,9 bis in connected mode. A stream of drilling fluid is pumped by primarypump 19 via the drill string 1 and the drill bit 7 into the annularspace 38 where drill cuttings are entrained into the stream. The streamthen flows in upward direction through the annular space 38 and via thedischarge conduit 48 and the discharge means 50 into the drilling fluidreservoir (not shown). The fluid pressure in the annular space 38 iscontrolled by controlling the pump rate of pump 19 and/or by controllingthe discharge means 50 and/or the tertiary pump 52.

[0025] When it is desired to remove the drill string from the borehole3, the individual drill strings joints are to be disconnected andremoved from the drill string 1 in sequential order. This is done bydisconnecting and removing the uppermost joint, moving the drill string1 upwardly to a position wherein the joint which is now the. uppermostjoint can be removed, etc. To remove the uppermost joint (i.e. drillstring section 10) the following procedure is followed. Rotation of thedrill string 1 by the top drive 18 is stopped while drilling fluid iscontinuously circulated through the drill string by operation of primarypump 19. The fluid chamber 20 is moved along support column 22 to aposition where the power tongues 34,36 are located at the level of theconnector 9 a,9 b, whereupon the tongues 34,36 are operated so as tobreak out and partly unscrew the connector 9 a,9 b. The connector 9 a,9b is unscrewed by the slips only to the extent that further unscrewingcan be done by the top drive 18. The fluid chamber 20 is then movedalong support column 22 so as to position connector 9 a,9 b inside thelower fluid chamber portion 27, and the seals 29 a,29 b are movedradially inward so as to seal against the respective upper and lowerdrill string sections 10,12. The secondary pump 30 is operated topressurise fluid camber 20. The top drive is then rotated in counterclockwise direction thereby further unscrewing the connector 9 a,9 b.Once the connector 9 a,9 b becomes disconnected the upper drill stringsection 10 is raised a short distance so as to position the upperconnector half 9 a in the upper portion 25 of the fluid chamber 20. Thevalve 32 is closed so as to seal the upper fluid chamber portion 25 fromthe lower fluid chamber portion 27. Simultaneously with closing thevalve 32 the primary pump 19 is stopped and the secondary pump 30 isoperated to pump drilling fluid through the fluid inlet 28 into thelower fluid chamber portion 27 and from there through lower drill stringsection 12 into the annular space 38. The seal 29 a is retracted toremove the upper drill string section, and the drill string joint whichhas now become the uppermost joint is connected to the top drive 18. Theprocedure described heretofore is repeated in order to remove the nowuppermost drill string joint. By the continued circulation of drillingfluid through the borehole 3 it is achieved that undesired settling ofparticles (e.g. drill cuttings) in the borehole occurs, and that thefluid pressure in the borehole can be controlled by controlling the pumprate of pump 30 and/or controlling the discharge means 50.

[0026] Instead of using the secondary pump 30 to pump drilling fluidthrough the lower drill string section 12 when the connector 9 a,9 b isdisconnected, the primary pump 19 can be used for this purpose in whichcase the primary pump 19 is connected to the fluid inlet 28 by suitableconduit means.

[0027] The above procedure relies on the use of the fluid chamber 20 tocontrol the fluid pressure in the borehole by continued fluidcirculation through the drill string 1 when the upper drill stringsection 10 is disconnected. In case it is impractical or impossible touse the fluid chamber an alternative procedure can be applied to connector disconnect the upper drill string section 10 to or from the drillstring 1. In the alternative procedure, which can be applied in theabsence of the fluid chamber, the tertiary pump 52 is operated so as topump drilling fluid through the circuit formed by the pump 52, thebranch connection 54, and the discharge means 50. By controlling thepump rate of pump 52 and/or by controlling the discharge means 50 thefluid pressure in the annular space 38 can be controlled. The non-returnvalve in the drill string 1 prevents flow of drilling fluid from theannular space 38 into the drill string 1. The alternative procedure canbe used, for example, in case drill string stabilisers prevent passageof the drill string through the fluid chamber.

[0028] An advantage of continued fluid circulation through the drillstring 1 using the fluid chamber 20 when the upper drill string jointare disconnected, is that the drilling fluid in the open part of theborehole 3 keeps flowing so that undesired settling of particles in theborehole is prevented. However once the drill string has been raised toa level whereby the drill bit 7 is located within the casing 42, thedrilling fluid which is pumped through the drill string 1 returns fromthe bit 7 through the annular space 38 to surface thereby leaving thedrilling fluid in the open part of the borehole 3 stationary. It istherefore preferred that, once the drill bit 7 is within the casing 42,pumping of drilling fluid by secondary pump 30 is stopped and pumping bytertiary pump 52 is commenced to control the fluid pressure in theborehole. This procedure has the advantage that the fluid chamber 20then is no longer required and can be removed from the drill string.

[0029]FIGS. 2 and 3 show the discharge means 50 in more detail. The flowof drilling fluid to be discharged is supplied to the discharge means bydischarge conduit 48.

[0030] The discharge means comprise two pressure chambers 60,61. Eachpressure chamber is provided with a membrane 62,63 made out of flexiblematerial, such as rubber. The membrane 62,63 divides each pressurechamber 60,61 in two compartments, a drilling fluid compartment 64,65and an expel fluid compartment 66,67. Both expel fluid compartments66,67 are interconnected by an expel fluid conduit 68 passing a controlvalve 69, which control valve 69 is a choke valve for controlling theflow of expel fluid through conduit 68 by throttling that flow.

[0031] The drilling fluid compartment 64,65 of each pressure chamber60,61 is provided with inlet valve means (70,71) to direct the drillingfluid to be discharged to the drilling fluid compartment 64 or 65respectively, and is provided with outlet valve means (72,73) to removedrilling fluid from the drilling fluid compartment 64 or 65respectively.

[0032]FIG. 2 shows a first mode of the discharge means and FIG. 3 showsa second mode.

[0033] In the first mode, as shown in FIG. 2, inlet valve 70 is open andinlet valve 71 is closed. Furthermore outlet valve 72 is closed andoutlet valve 73 is open. The flow of drilling fluid is indicated witharrows 75. From conduit 48 the drilling fluid flows to drilling fluidcompartment 64, whereby the membrane 62 is moved upwardly. Thereforeexpel fluid is expelled from compartment 66 through conduit 68 to expelfluid compartment 67, thereby passing choke valve 69. The flow of expelfluid is indicated with arrows 76. The inflow of expel fluid intocompartment 67 moves the membrane 63 downward, expelling the drillingfluid from compartment 65, which drilling fluid can be furthertransported, for example to a filtering system (not shown).

[0034] The flow of drilling fluid to compartment 64 is controlled bycontrolling choke valve 69 up to the moment that drilling fluidcompartment 64 is completely filled with drilling fluid. At that momentthe discharge means are shifted to the second mode as shown in FIG. 3.

[0035] In the second mode, as shown in FIG. 3, inlet valve 70 is closedand inlet valve 71 is open. Furthermore outlet valve 72 is open andoutlet valve 73 is closed. The flow of drilling fluid is indicated witharrows 75. From conduit 48 the drilling fluid flows to drilling fluidcompartment 65, whereby the membrane 63 is moved upwardly. Thereforeexpel fluid is expelled from compartment 67 through conduit 68 to expelfluid compartment 67, thereby passing choke valve 69. The flow of expelfluid is indicated with arrows 76. The inflow of expel fluid intocompartment 66 moves the membrane 62 downward, expelling the drillingfluid from compartment 64, which drilling fluid can be furthertransported, for example to a filtering system (not shown).

[0036] During operation of the discharge means the first and the secondmode will alternate with each other, whereby the choke valve 69 may bemaintained in the same position to achieve a predetermined resistance inexpel conduit 68 in both modes. That will result in a constantresistance for the drilling fluid passing the discharge means. Bychanging the position of the choke valve 69 that resistance will bechanged.

1. A drilling system for drilling a borehole into an earth formation,the drilling system comprising pump means for pumping drilling fluidinto the borehole and discharge means for discharging drilling fluidfrom the borehole, wherein the discharge means comprises two or morepressure chambers arranged to be alternately filled with the drillingfluid from the borehole for temporarily accommodating drilling fluidbeing discharged from the borehole, and control means for controllingthe fluid inflow into each pressure chamber, wherein the pressurechambers contain an expel fluid which is arranged to expel from a firstone of the pressure chambers by the inflow of drilling fluid in thatpressure chamber, into another of the pressure chambers, and wherein thecontrol means is arranged to control the outflow of expel fluid from thefirst pressure chamber.
 2. The drilling system of claim 1, wherein saidcontrol means is arranged to control the fluid pressure in the pressurechamber.
 3. The drilling system of claim 1, wherein the first and theother said pressure chambers are interconnected by an expel fluidconduit for transporting the expel fluid between the pressure chambers.4. The drilling system of claim 3, wherein the control means comprises acontrol valve in the expel fluid conduit.
 5. The drilling system ofclaim 4, wherein the control valve is a choke valve for controlling theresistance in the expel fluid conduit.
 6. The drilling system of claim1, wherein the control means comprises a controllable throttle opening.7. The drilling system of claim 3, wherein each pressure chamber isprovided with first and second compartments separated by a movable wall,whereby one of the compartments is filled with drilling fluid and theother compartment contains said expel fluid.
 8. The drilling system ofclaim 7, wherein said movable wall includes a flexible membrane.
 9. Thedrilling system of claim 7, wherein the compartments for expel fluid areinterconnected by the expel fluid conduit for transporting the expelfluid between said compartments, and wherein said compartments togetherwith the expel fluid conduit and the control means form a closed system.10. The drilling system of claim 7, wherein the system further comprisesinlet valve means to direct the drilling fluid to be dischargedalternately to one of said drilling fluid compartments and with outletvalve means to remove drilling fluid from the other drilling fluidcompartment.
 11. A method of drilling a borehole into an earthformation, whereby drilling fluid is pumped into the borehole andwhereby drilling fluid is discharged from the borehole and transportedto two or more pressure chambers, which are alternately filled with thedrilling fluid from the borehole whereby an expel fluid is expelled froma first one of the pressure chambers by the inflow of drilling fluid inthat pressure chamber, into another of the two or more pressurechambers, whereby the inflow of drilling fluid into the first pressurechamber is controlled by controlling the outflow of expel fluid which isexpelled from the first pressure chamber by the inflow of drilling fluidin that pressure chamber.
 12. The method of claim 11, whereby saidinflow of drilling fluid is controlled by controlling the fluid pressurein the pressure chamber.
 13. The method of claim 11, wherein thetransport the expel fluid from one pressure chamber to the otherpressure chamber is controlled by a control valve.
 14. The method ofclaim 11, whereby the inflow of drilling fluid into each of the pressurechambers is controlled.
 15. The method according to of claim 11, wherebyeach provided with a membrane to form a drilling fluid compartment andan expel fluid compartment, both having a variable content, the expelfluid compartments being interconnected by the expel fluid conduitwhereby drilling fluid to be discharged is alternately directed to oneof said drilling fluid compartments, while drilling fluid from the otherdrilling fluid compartment is removed.